Polyrotaxane is comprised of pseudopolyrotaxane, which comprises a linear molecule (axis) and cyclic molecules (rota) in which the linear molecule is included in cavities of cyclic molecules in a skewered manner, and capping groups, each of which locates at each end of the pseudopolyrotaxane (each end of the linear molecule) in order to prevent the dissociation of the cyclic molecules. For example, a polyrotaxane having α-cyclodextrin (hereinafter cyclodextrin may be simply abbreviated as “CD”) as cyclic molecules, and polyethylene glycol (hereinafter may be abbreviated as “PEG”) as a linear molecule has been intensively studied in recent years for its various characteristics.
Patent Document 2 discloses a compound comprising crosslinked polyrotaxane having characteristics as so-called slipping or sliding gels or a viscoelastic material. Particularly, Patent Document 2 discloses a specific crosslinked polyrotaxane, wherein a molecule of polyrotaxane comprises α-CD molecule as a cyclic molecule and PEG as a linear molecule included in the cyclic molecule, and molecules of polyrotaxane are crosslinked (bonded) to each other through chemical bonding.
However, a polyrotaxane, in which a linear molecule, PEG, is included in cyclic molecules, CD molecules, is insoluble in most solvents including water, and soluble only in dimethylsulfoxide (hereinafter, abbreviated as DMSO) and an alkaline aqueous solution when the linear molecule has a molecular weight of 10,000 or more. Accordingly, when preparing a crosslinked polyrotaxane, a solution of polyrotaxane in the solvent described above is used as a raw material. But, in order to use a crosslinked polyrotaxane stably, after preparing a crosslinked polyrotaxane, a solvent used in preparation, i.e., DMSO or an alkaline aqueous solution, must be replaced by pure water or saline. There is, however, a problem that optical characteristics, especially transparency of the crosslinked polyrotaxane are worsened at this time.    Patent Document 1: Japanese Patent No. 2810264.    Patent Document 2: WO 01/83566.